1. Field of the Invention
This invention relates to an alumina ceramic containing additions of MnO.sub.2 and TiO.sub.2 and of zirconia, of yttria-stabilized zirconia or of both; the ceramic can be one in which particles of zirconia and yttria-stabilized zirconia are in the metastable tetragonal or monoclinic crystal structure, or in both structures.
2. The Prior Art
Various alumina ceramics with additions of zirconia have been suggested. For example, U.S. Pat. No. 4,298,385, Claussen, discloses that additions of zirconia or hafnia to alumina increase the fracture toughness. It has been found, however, that, at high zirconia contents, it is difficult to retain a sufficient amount of the zirconia in the metastable tetragonal structure, and that the zirconia particles must be smaller than 0.5 .mu.m.
Another U.S. Pat. No. 4,316,964, Lange, discloses alumina ceramics with additions of zirconia, which can be stabilized with one or more of Y.sub.2 O.sub.3, CeO.sub.2, La.sub.2 O.sub.3, and Er.sub.2 O.sub.3 for the purpose of stabilizing a reasonable amount of the zirconia in the metastable tetragonal symmetry.
Still another U.S. Pat., No. 4,533,647, Tien, discloses modified alumina-zirconia composites. In one modification, chromia is present in solid solution in the alumina to increase the hardness and elastic modulus of the "matrix". In another modification, hafnia is used with the zirconia for the purpose of increasing the temperature at which the tetragonal to monoclinic transformation occurs, and, supposedly, as a consequence, to increase the fracture toughness, although it was found later that the increase in fracture toughness does not necessarily occur.
Further, U.S. Pat. No. 4,552,852, Manning, discloses alumina ceramics with zirconia or hafnia additions and a glass-phase. The material shows an improvement in thermal shock resistance.
Also, U.S. Pat. No. 4,587,225, Tsukuma, discloses composite ceramics of alumina and zirconia to which yttria additions have been made. The ceramic is produced by hot pressing, which enables the use of both lower temperatures and shorter times than would otherwise be required. This patent claims a high-strength material which is probably achieved, in part, as a consequence of the hot pressing.
A high strength metal working tool is disclosed by U.S. Pat. No. 4,666,467, Matsumoto et al. The tool is made from a sintered material comprising 50 to 98 percent of zirconia containing 1.5 to 5 m/o of yttria and 50 to 2 percent of alumina or spinel.
A method for producing zirconia bodies partially stabilized with yttria and including a secondary stabilized phase is disclosed by U.S. Pat. No. 4,659,680, Guile. The method involves sintering a shaped batch, cooling the shape quickly to a temperature within the range of 1000.degree. to 1475.degree., holding the shape at a temperature within the indicated range to cause precipitation of tetragonal ZrO.sub.2 as islands within cubic ZrO.sub.2 grains, and cooling to room temperature.
In addition, a fired ceramic material consisting essentially of Al.sub.2 O.sub.3 as the principal ingredient, 5-35 percent of ZrO.sub.2 as a first additive, and 0.25-5 percent of TiO.sub.2 and MnO.sub.2 as second and third additives, is disclosed by U.S. Pat. No. 4,760,038, Kinney et al. The additives, the reference states, "increase the thermal shock resistance of the fired ceramic composition, with retention of elevated temperature strength properties." (lines 7 and following of the abstract)
Further, Cutler, Bradshaw, Christensen and Hyatt have disclosed that 96 percent alumina bodies containing small additions of MnO.sub.2 and TiO.sub.2 have been produced at sintering temperatures in the range of 1300.degree. to 1400.degree., Journal of the American Ceramic Society, Volume 40, No. 4, April, 1957, pages 134 et seq.
Finally, U.S. Pat. No. 3,686,007, Gion, discloses the use of a mixture of TiO.sub.2 and MnO.sub.2 as a flux in alumina ceramics containing relatively large amounts of ball clay and feldspar.